drm: bridge: ldb: Warn if LDB clock does not match requested link frequency

The LDB clock needs to be exactly 7-times the pixel clock used by the
display.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208065538.1753666-1-alexander.stein@ew.tq-group.com